2 Answers
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The root partition of an EC2 instance is taken directly from the AMI you're launching with, and it is the same size as the AMI itself. If you have a 30GB AMI, you get a 30GB root partition.

The "instance storage" described on that page comes as one or more separate scratch partitions. I don't have a running instance in front of me, but the partitions will typically show up as /dev/sdb, /dev/sdc, etc. I believe that the first is formatted as ext3 and mounted at /mnt; you have to mount (and possibly format) the other partitions yourself.

There was no seperate partition added, or mounted! Automatically And adding it myself I'm not sure if I get charged for it! You got a aws account? Try it and let me know if it's the same for you too?
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001Jun 27 '10 at 22:53

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I just started an Large Ubuntu Lucid instance using the AMIs from uec-images.ubuntu.com/releases/lucid/release. /dev/sdb is a 414G drive formatted as ext3 and mounted at /mnt, and /dev/sdc is an unpartitioned 414G drive (See my edits to my original answer - I can't get any formatting here, apparently)
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Evan BroderJun 28 '10 at 14:10

The 850GB you reference is called ephemeral storage (or instance store), and the size depends on the instance type. It gets destroyed if you shut down your instance, while EBS volumes do not ( you didn't specify which type of boot partition you are using: ephemeral or EBS).

The image itself determines how much of the 850GB instance store is mounted and to what device. If only 30GB shows up, that's because the image was created with that storage mounted.

You can mount the rest of the storage by listing the scsi devices like ls -l /dev/sd* Then partition the ephemeral device using fdisk, then format it with mkfs, then mount it with mount.